In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. In particular, the change-over from i-line (365 nm) to shorter wavelength KrF laser (248 nm) brought about a significant innovation, enabling mass-scale production of 0.18 micron rule devices. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
Resist materials adapted for KrF excimer lasers enjoyed early use on the 0.3 micron process, went through the 0.25 micron rule, and currently entered the mass production phase on the 0.18 micron rule. Engineers have started investigation on the 0.15 micron rule, with the trend toward a finer pattern rule being accelerated. A wavelength change-over from KrF to shorter wavelength ArF laser (193 nm) is expected to enable miniaturization of the design rule to 0.13 μm or less. Since conventionally used novolac resins and polyvinylphenol resins have very strong absorption in proximity to 193 nm, they cannot be used as the base resin for resists. To ensure transparency and dry etching resistance, some engineers investigated acrylic and alicyclic (typically cycloolefin) resins as disclosed in JP-A 9-73173, JP-A 10-10739, JP-A 9-230595 and WO 97/33198. With respect to F2 excimer laser (157 nm) which is expected to enable further miniaturization to 0.10 μm or less, more difficulty arises in insuring transparency because it was found that acrylic resins are not transmissive to light at all and those cycloolefin resins having carbonyl bonds have strong absorption. It was also found that polyvinylphenol is somewhat improved in transmittance in proximity to 160 nm, but far below the practical level, and reducing carbonyl and carbon-to-carbon double bonds is essential for insuring a transmittance. However, cyclic structures and carbon-to-carbon double bonds greatly contribute to an improvement in dry etching resistance. A polymer for use with an ArF excimer laser, in which a benzene ring is excluded and instead, an alicyclic structure is introduced for improving etching resistance, is difficult to provide transparency since it acquires solubility by relying on carboxylic acid.
The use of a fluorine-substituted polymer was found effective as a means for improving transparency. Making a study to improve the transparency of an acrylic polymer used in ArF resist compositions, the inventor proposed the use of an acrylic derivative having a fluorine-substituted backbone.
In most cases, dry etching resistance is conventionally discussed in conjunction with the selection ratio of etching. As described in many reports, for example, J. Photopolymer Sci. and Technol., Vol. 5, No. 3 (1992), p. 439, J. Electrochem. Soc.: Solid-State Sci. and Technol., Vol. 130, No. 1, January 1983, p. 143, and SPIE, Vol. 2724, p. 365 (1996), engineers attempted to express the dry etching selection ratio of a single layer resist using various parameters. Typical are Onishi parameter and ring parameter.
It was recently reported in SPIE, Vol. 3678, p. 1209 (1999) that micro-roughness develops on the resist surface after dry etching and is transferred after substrate processing and resist removal. Making extensive studies, the inventor found that the development of roughness after etching occurs when dry etching of SiO2 is carried out with a fluorocarbon gas such as CF4, CHF3, C2F6, C3F8 or C4F10 and that roughness increases under the high throughput conditions where the RF power is increased for high etching selection ratio, that is, fast etching of oxide film. It was further found that roughness largely differs depending on the type of polymer used in ArF single layer resist. A noticeable roughness develops with acrylic polymers. In contrast, roughness declines with cycloolefin polymers such as norbornene homopolymers and alternating copolymers of norbornene with maleic anhydride. In particular, norbornene homopolymers give small values of roughness even compared with polyhydroxystyrene for KrF. Herein, acrylic polymers with pendant adamantane exhibit a satisfactory value of etching speed, that is, selection ratio, fully comparable to cycloolefin polymers. When high selectivity etching was effected in an etching speed ratio of at least 3 between oxide film and resist, the surface roughness Rms of the etched surface as measured by atomic force microscopy (AFM) was more than 15 nm for acrylic polymers and less than 3 nm for cycloolefin polymers. These results indicate that the selection ratio of etching does not necessarily coincide with the roughness after etching.
It is pointed out that what becomes a problem as a result of wavelength reduction is a lowering of transparency, and in the case of a positive resist material, a negative working phenomenon that the exposed areas become insoluble as the dose of exposure is increased. Those portions which have turned negative are insoluble not only in alkali developers, but also in organic solvents such as acetone. This indicates that gel forms as a result of crosslinking of molecules together. Radical generation is probably one cause of crosslinking. As a result of wavelength reduction, the exposure energy is increased so that even C—C bonds and C—H bonds may be excited in the case of F2 exposure (157 nm). As a result of excitation, radicals are generated with a possibility that molecules are bonded together. For polymers having an alicyclic structure for use in ArF exposure, for example, polynorbornene, an outstanding negative working phenomenon was observed. It is believed that these polymers have a structure susceptible to crosslinking since the alicyclic group has many C—H bonds at the bridgehead. On the other hand, it is well known that α-methylstyrene and derivatives thereof are effective for preventing crosslinking. Alpha-methylstyrene can mitigate the negative working phenomenon, but fail to completely eliminate the phenomenon. Moreover, since oxygen absorption is considerable in the VUV region, exposure is effected under the conditions that oxygen is purged, with an inert gas such as nitrogen or argon, to an oxygen concentration of 1 ppm or lower. Since oxygen is an effective radical trapping agent, this means that the radicals generated have a long lifetime and more crosslinking takes place.